refractive index & dielectric constant

[Kolahal Bhattacharya]

We know,n=(rel.permittivityxrel.permeability)^(1/2)
For natural transperant mediums (to visible light of EM spectrum)are non-magnetic.So, rel.permeability=1
hence n=(dielectric constant)^(1/2)
Book says this is not valid for all materials,as dielectric constant is frequency dependent.Why yhis is not the case with vacuum?
As far as I know, we are familiar to r.i. s which are measured at the frequency of visible light.Book says dielectric constant are measured at much lower frequency.Why?why not we measure it at optical frequency?Wikipedia says static dielectric constant is a good approx.for altering field of low frequency--why?

[Andrew Mason]

We know,n=(rel.permittivityxrel.permeability)^(1/2)
For natural transperant mediums (to visible light of EM spectrum)are non-magnetic.So, rel.permeability=1
hence n=(dielectric constant)^(1/2)
Book says this is not valid for all materials,as dielectric constant is frequency dependent.Why yhis is not the case with vacuum?
As far as I know, we are familiar to r.i. s which are measured at the frequency of visible light.Book says dielectric constant are measured at much lower frequency.Why?why not we measure it at optical frequency?Wikipedia says static dielectric constant is a good approx.for altering field of low frequency--why?There are no electric dipoles in empty space.

It is the electric dipoles (polar molecules) in the dielectric which reduce the applied field, but these do not respond quickly enough at light frequencies (the polar molecules can't turn quickly enough). So the dielectric strength has to be measured at lower frequencies.

AM